Quantifying the evil of dictators can be a tricky business. Most would quantify the horrors of dictators by how many people were killed under his rule. In that case, there were several dictators worse than Hitler.
Mao Zedong - Communist dictator of China - 25–30 million died.
Josef Stalin - Communist dictator of USSR - 30 million died.
Leopold II - Colonial leader of Belgium - had upwards of 20 million Congolese salughtered.
At this point the numbers vary between Pol Pot, Saddam Hussein, Kim Il Sung, Idi Amin, Ho Chi Minh. Anywhere between 1–10 million range.